Method for purification of wastewater

ABSTRACT

A method of reducing nitrogen, phosphor, and excessive biological oxygen demand (BOD) in wastewater supplied to and passing through a granular filter bed activated with bacteria includes in the flow direction of the wastewater through the bed simultaneously precipitation of phosphorous, denitrification and reduction of the excessive BOD in a single process step. The reduction of excess BOD is produced by blowing in an oxygen-containing gas, preferably air. Immediately before the wastewater enters the filter bed a precipitation/flocculation agent may be added for precipitation of phosphor.

BACKGROUND OF THE INVENTION

This invention relates to a method of purification of waste water bytreatment in a granular filter bed that has been activated by bacteria,preferably heterotrophic bacteria.

It is known in the art to remove nitrogen and phosphor simultaneouslyfrom wastewater with the use of an active sludge that contains aerobicbacteria. It is also known from the Swedish patent No 456 990 to add thesludge in anaerobic conditions, so that the aerobic bacteria are urgedto liberate phosphor, which the anaerobic bacteria can then take up. Theprocess takes place in treatment basins under heavy agitation.

It is known from the French patent No 2619 804 to denitrify drinkingwater using a granular filter bed, to which are added sources ofphosphor and carbon. Here, the majority of the phosphor is taken up byanaerobic bacteria.

BRIEF SUMMARY OF THE INVENTION

It is an object of this invention to provide a method incontradistinction to the known art cited above, wherein wastewater istreated, in a granular filter bed that has been activated by bacteria,preferably heterotrophic bacteria. According to the invention, therethen occurs simultaneous precipitation of phosphor and denitrificationin the wastewater during the first part of its percolation through thefilter bed, and later on in the bed an excess biological oxygen demand(BOD) is reduced, preferably close to zero.

It should be noted that before the mentioned inventive method isapplied, the water has been first subjected to nitrification accordingto the known art, such that the nitrogen compounds in the wastewaterhave been converted to nitrates or to nitrites.

DETAILED DESCRIPTION

In a preferred method according to the invention, the BOD reductiontakes place with the addition of oxygen, and, especially, in a simpleand advantageous method, by blowing an oxygen-containing gas, preferablyair.

A precipitation/flocking agent for precipitating phosphor is added tothe wastewater immediately before the latter enters the filter bed.

The quantity of organic substances in wastewater that can function assources of carbon for the bacterium strain is often not enough toprovide sufficient denitrification. It may be necessary to add a furthersource of carbon, cheap hydrocarbons such as methanol, starch ormolasses being selected for this purpose. These carbon sources maysuitably be added to the wastewater immediately before it enters thefilter bed. It is probable that the founding strain of bacteria adhereto the grains, usually sand, of the filter bed. Here they come intointimate contact with the carbon source, and nitrates present arereduced to nitrogen gas. A typical formula for such denitrification is:

    NO.sub.3 +1.08 CH.sub.3 OH+H.sup.+ =0.065C.sub.5 H.sub.7 O.sub.2 N+0.47N.sub.2 +0.76CO.sub.2 +2.44 H.sub.2 O

where 0.065 C₅ H₇ O₂ N corresponds to the production of new bacteria.

If the amount of sludge is increased by relatively large and voluminousprecipitates or floccules of phosphor, e.g. iron phosphate, then it isreasonable to assume that denitrification will be disturbed unfavorablyby these precipitations. However, no negative effect at all by thephosphates on the denitrification process in different types of filterbed has been found. There thus exists the basic condition for enablingsimultaneously the reduction of nitrogen, precipitation of phosphor,reduction of excessive BOD and filtration in a single process step,which requires two or three steps according to the known prior art.

Iron chloride is a suitable precipitation agent for phosphor. Othersuitable precipitation/flocculation agents in use.. e.g. aluminumsulphate with or without polyeletrolyte additives, lime etc, have beenfound not to affect the denitrification process either.

A factor supposed to affect denitrification is washing the filter bed.Practically, it is quite possible to wash the bed so thoroughly that thebacterium strain is unfavorably reduced. Heavy counter-flushing of astationary filter appears to be particularly injurious. However, it hasbeen found that the washing sufficient for removing sludge andcontaminants from the sand does not notably impoverish the bacteriumstrain. All the nitrogen gas that may be entrained in the sand andsludge is also removed in such washing. The so-called continuous sandfilters are particularly favorable for use in a method according to thepresent invention, only a small portion of the filter bed at a timebeing subjected to such washing. In these sand filters, where mostcontaminated sand is taken away for washing, and then returned to thefilter bed, there are great possibilities for regulating both theintensity and periodicity of washing, such as sand filter, which is verysuitable for denitrification and purification from phosphor, asdescribed in the Swedish patent specification No 7602999-0.

Denitrification is directly proportional to the total surface of thesand grains in the filter bed. Its capacity may be improved by reducingthe size of the grains and/or by increasing its height. At the sametime, the period of active residence between washings of each grain inthe bed can thus be extended also.

The result obtained, i.e. nitrogen and phosphor reductions, is mainlycontrolled by the added quantities of carbon source and phosphorprecipitation agents. By a well-balanced selection of these factors, asimultaneous nitrogen and phosphor reduction of over 90% may be obtainedfor a surface load on the filter bed of 15 m/h.

In the practical operation of a filter described above, there may belarge variations in the composition of the incoming wastewater. Theamount of sludge as well as the nitrogen and phosphor contents may varygreatly within relatively short periods of time. The addition of carbonsource should not be of the order of magnitude enabling an unnecessarilylarge BOD load to be achieved in the departing water, neither should itbe too low for achieving an optimum result.

Continuous monotoring of the incoming and/or outgoing water with respectto the content of nitrogen, phosphor and its BOD/COD(biological/chemical oxygen demand) can be arranged to control, inaccordance with the known art, the amounts of the additives such as toobtain optimum conditions.

In a granular filter bed, denitrification and phosphor precipitation areboth rapid, compared with corresponding processes in a basin. Theresidence time for wastewater in a continuously operating filter bed,for example, is of the order of magnitude 10 min, compared with severalhours in a basin. This makes it particularly advantageous, in carryingout simultaneous nitrogen and phosphor reduction in a continuouslyoperating filter bed, to regulate the additions of carbon source and/orprecipitating agent on the basis of measured values of nitrogen and/orBOD/COD, preferably in the outgoing wastewater. This control of theadditives can also be used to advantage when the invention is applied tothe stationary filter beds.

In certain cases it may, however, be suitable to dispense withregulation of the carbon source, and knowingly add excessive amountsthereof instead, thus creating a high BOD. But even with regulation ofthe carbon source additive, it would in most cases be impossible to meetthe ever more severe requirements for low BOD in the outgoing wastewaternow being demanded. The invention thus solves the problems, not onlywith respect to the simultaneous phosphor precipitation anddenitrification, but also the invention, the reduction of BOD excess isaccomplished by blowing an oxygen-containing gas, usually air, into theupper part of a filter bed. This may be performed by blowing the gasthrough a row of nozzles or jets, for example.

I claim:
 1. A method of reducing nitrogen, phosphor and excessive biological oxygen demand (BOD) in wastewater, comprising:supplying said wastewater to a granular filter bed for passing said wastewater therethrough; activating said granular filter bed with bacteria; and in the flow direction of said wastewater through said filter bed simultaneously precipitating said phosphor, reducing said nitrogen and reducing said excessive BOD in a single process step.
 2. The method as claimed in claim 1 and further comprising:adding oxygen to said filter bed for reducing said excessive BOD.
 3. The method as claimed in claim 2 wherein:said adding of said oxygen comprises blowing an oxygen-containing gas into said filter bed.
 4. The method as claimed in claim 3 and further comprising:blowing said oxygen-containing gas into an upper part of said filter bed.
 5. The method as claimed in claim 4 wherein:said oxygen-containing gas comprises air.
 6. The method as claimed in claim 3 wherein:said oxygen-containing gas comprises air.
 7. The method as claimed in claim 3 and further comprising:immediately before said wastewater enters said filter bed adding to said filter bed a precipitation/flocculation agent for precipitating said phosphor.
 8. The method as claimed in claim 2 and further comprising:immediately before said wastewater enters said filter bed adding to said filter bed a precipitation/flocculation agent for precipitating said phosphor.
 9. The method as claimed in claim 1 and further comprising:immediately before said wastewater enters said filter bed adding to said filter bed a precipitation/flocculation agent for precipitating said phosphor.
 10. The method as claimed in claim 9 and further comprising:monitoring outgoing wastewater from said filter bed for determining the phosphor content thereof; and controlling the amount of said precipitation/flocculation agent added in response to said phosphor content determined by said monitoring.
 11. The method as claimed in claim 1 and further comprising: in addition to the amount of organic material in the wastewater functioning as a carbon source, adding a further source of carbon to said filter bed for reduction of said nitrogen.
 12. The method as claimed in claim 11 and further comprising:adding said further source of said carbon to said wastewater immediately prior to the entry of said wastewater into said filter bed.
 13. The method as claimed in claim 12 and further comprising:monitoring said outgoing wastewater from said filter bed for determining the nitrogen content thereof; and controlling the amount of carbon added from said further source of carbon in response to said nitrogen content determined by said monitoring.
 14. The method as claimed in claim 11 and further comprising:monitoring outgoing wastewater from said filter bed for determining the nitrogen content thereof; and controlling the amount of carbon added from said further source of carbon in response to said nitrogen content determined by said monitoring.
 15. The method as claimed in claim 11 and further comprising:monitoring outgoing wastewater from said filter bed for determining the value of biological/chemical oxygen demand (BOD/COD) therein; and controlling said addition of carbon from said further source of carbon in response to said determined BOD/COD value.
 16. The method as claimed in claim 11 and further comprising:monitoring outgoing wastewater from said filter bed for determining the phosphor content thereof; and controlling the amount of said precipitation/flocculation agent added in response to said determined said phosphor content determined by said monitoring.
 17. The method as claimed in claim 16 and further comprising:monitoring outgoing wastewater from said filter bed for determining the value of biological/chemical oxygen demand (BOD/COD) therein; and controlling said addition of carbon from said further source of carbon in response to said determined BOD/COD value.
 18. The method as claimed in claim 11 and further comprising: in addition to the amount of organic material in the wastewater functioning as a carbon source, adding a further source of carbon to said filter bed for reduction of said nitrogen.
 19. The method as claimed in claim 18 and further comprising:blowing said oxygen-containing gas into an upper part of said filter bed.
 20. The method as claimed in claim 1 and further comprising:operating said filter bed continuously. 